1. Field of the Invention
The present invention relates to the field of dc/dc power converters, such as switched-mode power supplies or linear voltage regulators, meant to supply a load with a regular voltage of fixed value. The present invention more specifically applies to the maintaining of the supply voltage of the load even in case of an abrupt current variation therein. For example, the present invention applies to power converters for supplying a microprocessor, the current surge of which may vary, at a relatively high recurrence frequency (from about a few hertz to a few kilohertz), between a few mA and 15 or 15 A, or even more.
2. Discussion of the Related Art
The increase in microprocessor performance requires power converters which have better and better voltage stability and respond more and more rapidly to a variation of the load. For example, a converter for supplying a microprocessor known under trade name "PENTIUM-PRO" of make "Intel" has to withstand a current variation from 0.3 A to 10 A in 350 ns with a voltage accuracy of 5%.
Several conventional solutions are used to improve the response of the converter to such abrupt current variations.
FIG. 1 shows an example of a conventional scheme for supplying a microprocessor. A dc/dc converter 1 receives a supply voltage Ve, for example of +12 volts or +5 volts, and provides a voltage Vin to a microprocessor 2. The value of voltage Vin (for example, from +2.1 volts to +3.5 volts) is generally set by microprocessor 2 which communicates with converter 1 via a digital link 3. Several chemical storage capacitors C are connected in parallel between supply line Vin and the ground to limit the transient variations of voltage Vin due to a variation of the current surged by microprocessor 2. One or several ceramic decoupling capacitors C' are generally connected between a supply terminal S of the microprocessor receiving voltage Vin and the ground. A disadvantage of this solution is that very high capacitance capacitors, which must further have very low equivalent series resistances and inductances, are very expensive. A conventional solution such as shown in FIG. 1 typically requires, for the above example of a "PENTIUM-PRO" microprocessor, ten chemical capacitors of 1,500 .mu.F, each having an equivalent series resistance of 44 m.OMEGA., to reach a capacitance higher than 4,000 .mu.F with an equivalent series resistance lower than 5 m.OMEGA. corresponding to the constraints set by the manufacturer. Further, the equivalent series inductance of a capacitor generally is around 10 mH.
Another solution, which is applicable if the power converter is a switched-mode supply (PWM), is to operate the switched-mode supply at high switching frequencies. If such a solution requires chemical capacitors with a lower capacitance between the converter and the microprocessor, their equivalent series resistances and inductances remain critical as concerns their cost. Further, the concept of a switched-mode supply operating at 500 kHz, or even more, requires the use of high-performance components, especially to stabilize the feedback loop while taking into account the behavior of the components and of the pattern of the printed circuit at such a high frequency.